Shock absorber

A shock absorber in United States parlance (sometimes damper in British use) is a mechanical device designed to smooth out or damp a sudden shock impulse and dissipate kinetic energy. It is analogous to the resistance in an electric RLC circuit.

Shock absorbers are an important part of automobile suspensions, aircraft landing gear, and the supports for many industrial machines. Large shock absorbers have also been used in architecture and civil engineering to reduce the susceptibility of structures to earthquake damage and resonance.

Applied to a structure such as a building or bridge it may be part of a seismic retrofit or as part of new, earthquake resistant construction. In this application it allows yet restrains motion and absorbs resonant energy, which can cause excessive motion and eventual structural failure.

In a vehicle, it reduces the effect of traveling over rough ground. Without shock absorbers, the vehicle would have a bouncing ride, as energy is stored in the spring and then released to the vehicle, possibly exceeding the allowed range of suspension movement. Control of excessive suspension movement without shock absorption requires stiffer (higher rate) springs, which would in turn give a harsh ride. Shock absorbers allow the use of soft (lower rate) springs while controlling the rate of suspension movement in response to bumps. They also, along with hysteresis in the tire itself, damp the motion of the unsprung weight up and down on the springiness of the tire. Since the tire is not as soft as the springs, effective wheel bounce damping may require stiffer shocks than would be ideal for the vehicle motion alone.

Pneumatic and hydraulic shock absorbers commonly take the form of a cylinder with a sliding piston inside. The cylinder must be filled with a viscous fluid, such as hydraulic fluid or air. This fluid filled piston/cylinder combination is a dashpot. Spring-based shock absorbers commonly use coil springs or leaf springs, though torsion bars can be used in torsional shocks as well. Ideal springs alone, however, are not shock absorbers as springs only store and do not dissipate or absorb energy. Vehicles typically employ both springs or torsion bars as well as hydraulic shock absorbers. In this combination, "shock absorber" (or simply "shocks") is reserved specifically for the hydraulic piston that absorbs and dissipates vibration. The springs are not termed "shock absorbers".

Shock absorbers must absorb or dissipate energy. One design consideration, therefore, that must be made when designing or choosing a shock absorber is where that energy will go. Commonly, as in most dashpots, that energy is converted to heat inside the viscous fluid. In hydraulic cylinders, the hydraulic fluid will heat up. In air cylinders, the hot air is usually exhausted to the atmosphere. In other types of dashpots, such as electromagnetic ones, the dissipated energy can be stored and used later.

There are several commonly-used approaches to shock absorption:

  • Hysteresis of structural material, for example the compression of rubber disks, stretching of rubber bands and bungie cords, bending of steel springs, or twisting of torsion bars. Elasticity is a form of hysteresis. The elastic material stretches or compresses when force is applied according to Hooke's law.
  • Dry friction as used in wheel brakes, but using disks (classically made of leather) at the pivot of a lever, with friction forced by springs. Used in early automobiles such as the Ford Model T, up through many of the British sports cars of the 1950s and early 1960s. Although now considered obsolete, an advantage of this system is its mechanical simplicity; the degree of damping can be easily adjusted by tightening or loosening the screw clamping the disks, and it can be easily rebuilt with simple hand tools.
  • Fluid friction, for example the flow of fluid through a narrow orifice (hydraulics), constitute the vast majority of automotive shock absorbers. An advantage of this type is that using special internal valving the absorber may be made relatively soft to compression (allowing a soft response to a bump) and relatively stiff to extension, controlling "jounce", which is the vehicle response to energy stored in the springs; similarly, a series of valves controlled by springs can change the degree of stiffness according to the velocity of the impact or rebound. Specialized shock absorbers for racing purposes may allow the front end of a dragster to rise with minimal resistance under acceleration, then strongly resist letting it settle, thereby maintaining a desirable rearward weight distribution for enhanced traction. Some shock absorbers allow tuning of the ride via control of the valve by a manual adjustment provided at the shock absorber. In more expensive vehicles the valves may be remotely adjustable, offering the driver control of the ride at will while the vehicle is operated. The ultimate control is provided by dynamic valve control via computer in response to sensors, giving both a smooth ride and a firm suspension when needed. Many shock absorbers contain compressed nitrogen, to reduce the tendency for the oil to foam under heavy use. Foaming temporarily reduces the damping ability of the unit. In very heavy duty units used for racing and/or off-road use, there may even be a secondary cylinder connected to the shock absorber to act as a reservoir for the oil and pressurized gas.
  • Compression of a gas, for example pneumatic shock absorbers, which can act like springs as the air pressure is building to resist the force on it. Once the air pressure reaches the necessary maximum, air dashpots will act like hydraulic dashpots. In aircraft landing gear air dashpots may be combined with hydraulic dampening to reduce bounce. Such struts are called "oleo" struts (combining oil and air).
  • Magnetic effects. Eddy current dampers are dashpots that are constructed out of a large magnetic inside of a non-metallic, conducting tube. Furthermore, many modern hybrid automobiles have regenerative braking, which uses a reversed electric motor to dampen and eventually stop the motion of the car.
  • Inertial resistance to acceleration, for example the Citroën 2CV has an additional pair of rear shock absorbers that damp wheel bounce with no external moving parts. The energy is absorbed by hydraulic fluid friction, but their operation depends on the inertia of an internal weight.
  • Composite hydropneumatic devices which combine in a single device spring action, shock absorption, and often also ride-height control, as in some models of the Citroën automobile.
  • Conventional shock absorbers combined with composite pneumatic springs with which allow ride height adjustment or even ride height control, seen in some large trucks and luxury sedans such as certain Lincoln automobiles. Ride height control is especially desirable in highway vehicles intended for occasional rough road use, as a means of improving handling and reducing aerodynamic drag by lowering the vehicle when operating on improved high speed roads.

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